Piezoelectricity describes the electric charge that accumulates within some solid materials (such as crystals, certain ceramics, and biological matter such as chitin, bone, DNA and various proteins) in response to an applied mechanical force. The word piezoelectricity means electricity resulting from pressure.
The piezoelectric effect is understood as the electromechanical interaction between the mechanical and the electrical states in crystalline and polycrystalline dielectric materials having no inversion symmetry. The piezoelectric effect is a reversible process, in that materials exhibiting the direct piezoelectric effect (the generation of electrical charge resulting from an applied mechanical force) also exhibit the reverse piezoelectric effect (the generation of a mechanical force resulting from an applied electrical field). For example, polyvinylidene fluoride (PVDF) polymeric crystals may generate measurable piezoelectricity when their static structure is deformed. Conversely, those same polymeric crystals will change their static dimensions when an external electric field is applied.
Pyroelectricity is the electrical response of a material to a change in temperature. The change in temperature modifies the positions of the atoms slightly within the crystal structure of a pyroelectric material, such that the polarization of the material changes. This polarization change gives rise to a voltage across the material. If the temperature remains constant, the pyroelectric voltage gradually disappears due to leakage current (the leakage can be due to electrons moving through the material, ions moving through the air, current leaking through surface contaminants or test equipment attached to the material, etc.) Very small changes in temperature (as small as 10−6° C.) can produce a measurable electric potential due to a material's pyroelectric properties.
Pyroelectric charge in polymers and minerals develops on the opposite faces of asymmetric crystals. The direction in which the propagation of the charge tends toward is usually constant throughout a pyroelectric material, but in some materials this direction can be changed by a nearby electric field. These pyroelectric materials are said to exhibit ferroelectricity—a spontaneous electric polarization that can be reversed by the application of an electric field. All pyroelectric materials are also piezoelectric. However, some piezoelectric materials have crystal symmetry that does not allow the pyroelectric effect to manifest.
Piezoelectric materials may be used for ultrasonic sensors. Ultrasonic sensors may be used, for example, in biometrics for detecting a fingerprint. However, these ultrasonic sensors do not generally utilize the pyroelectric characteristics of the piezoelectric material.